1. Field of the Invention
The present invention relates to a digital audio tape recorder for recording a sound signal as digital data on a magnetic tape by using a rotary head and, more particularly, to a digital audio tape recorder which can cope with a change of a sampling frequency.
2. Description of the Related Art
With the progress of digital technique, processing of a large capacity of data is enabled and digital recording is adopted for recording various signals. In the field of sound signal recording, compact disks and the like which adopt digital recording have become widespread.
Among these apparatuses which utilize digital recording, digital audio tape recorders (DAT) which are capable of not only reproducing but also recording a sound signal have attracted attention.
DAT's are advantageous in that since a sound signal is recorded on a magnetic tape as digital data, they are free from the problems such as wow flutter, hiss noise and modulation noise, which are inevitable in analog recording, and in that since they have a wide dynamic range which can realize flat frequency characteristics in a wide frequency, recording and reproduction with a high sound quality is realized.
There are two types of DAT's. One utilizes a rotary head, and the other utilizes a fixed head. DAT's using a rotary head have been standardized and increasingly produced as manufactured goods.
In a DAT of a rotary head system, a signal is stored in each track inclined at a little over 6.degree. with respect to the direction of travel of the magnetic tape, as shown in FIG. 6.
Each track is separated into an area for recording sub codes consisting of various kinds of information necessary for reproduction, an area for recording an ATF signal for tracking, etc. as well as an area for recording digital sound data (PCM).
The rotary head is provided with two magnetic heads for tracing tracks of the magnetic tape and two tracks are traced in one revolution of the rotary head. The magnetic tape is so designed as to come into contact with the rotary head which rotates at a high speed only in the range of 90.degree..
Consequently, sound data are intermittently recorded and reproduced on and from the magnetic tape by the magnetic head, and in order to input and output continuous sound, it is necessary to convert the time axis of data.
The DAT adopts an interleave format for recording data in dispersion in order to suppress a random error or a burst error which is produced in recording data on the magnetic tape to the minimum.
In order to convert the time axis and record and reproduce data in the interleave format, RAM for storing a considerable amount of data is necessary.
In recording data, after the time axis is converted, data for two tracks are written in the RAM in the interleave format and while the data for the next two tracks are written, the data for the two tracks stored in the RAM are read out and recorded on the magnetic tape. In reproducing the data, the data read out of the magnetic tape is temporarily written into the RAM and after de-interleaving the data read out of the RAM, the time axis is converted.
The conversion between an analog sound signal and a digital sound signal is carried out by an A/D converter and a D/A converter. In DAT's, three sampling frequencies, namely, 48 kHz, 44.1 kHz and 32 kHz are prepared as a sampling frequency for sampling the digital data from a sound signal in the A/D converter, thereby enabling a long-time recording and direct transmission and reception of the digital signal to and from another audio apparatus.
Since one of these frequencies is adopted in recording or reproduction, the RAM has a capacity which can cope with the amount of data output when the sampling frequency is the highest and the same RAM copes with all the sampling frequencies.
In such a DAT, when a sound signal is recorded while reading the data which is stored in the RAM as described above and writing the data into the magnetic tape, if the sampling frequency is low, the amount of data is small and there is an area in a part of the RAM in which the data is not overwritten. As a result, if data is recorded at a low sampling frequency after reproduction and recording at a high sampling frequency is carried out, the data which is not overwritten remains in the RAM and this data is inconveniently recorded on the magnetic tape.
To prevent this, in a conventional DAT, when the sampling frequency is changed or the reproduction mode is changed into the recording mode, null data (0) is written into the entire part of the RAM while utilizing the ordinary RAM writing time, and after the contents of the RAM are cleared, data is written.
In this system, if the RAM is cleared at the start of recording, it is impossible to record data while the RAM is being cleared. It is therefore impossible to record in the RAM the data supplied during the period from the time when the command of the start of recording is issued to the time when the clearing of the RAM is finished. In other words, the sound data supplied during this period is discarded.
In the case of adopting the interleave recording system in which recording is completed by recording data in two tracks, since the data for two tracks are first written into the RAM and the actual recording of the data on the magnetic tape is begun when the data for the next two tracks are input, the starting of recording data on the magnetic tape is further delayed.
The rotary head having two magnetic heads rotates at 2,000 rpm. One revolution takes about 30 msec and in this period the writing or reading of data for two tracks is finished.
Therefore, the RAM is cleared in the first 30 msec from the issue of the command of recording, and the data itself is discarded. In the next 30 msec, data is written into the RAM and the recording of the data on the magnetic tape is started after these processings are finished, namely, 60 msec after the issue of command of recording.
That is, the data for 30 msec is discarded and the recording of data on the magnetic tape is started 60 msec after the issue of command of recording.
In an ordinary operation, recording is often continued in the same mode (at the same sampling frequency). In other words, it is often the case that the clearing of the RAM before the start of recording is unnecessary.
Since it is impossible to record data while the RAM is being cleared, it is impossible to record in the RAM the data supplied during the period from the time when the command of the start of recording is issued to the time when the clearing of the RAM is finished. In other words, the sound data supplied during this period is discarded. It is therefore demanded to avoid unnecessary RAM clearing as much as possible.